Automatic inverse feedback method and means for correcting response characteristic of phototube circuits



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N. R. AUTOMATIC INVERSE FEEDBACK METHOD AND MEANS FOR CORRECTING RESPONSE CHARACTERISTIC l OF PHOTOTUBE CIRCUITS Filed oct. 18, 194e sheets-sheet 2 VOLT-$ NORMA/vf?. Gu/vomso/v IN1/EN ToR.

Patented Dec. .19, 1950 vos QltoRRECTINGRESPNS ACT ms'mc oF lIHoflrozluiali cm- Norman 1R. G'nnerson, Pasadena, Calif. .Annoat'ion october 18, 19218, SealNofS'sme This intention 'pertains vto AAnietnoi's 'ani arrrangernents whereby 'an Aeleo'trioa'l oir-enit anparatueeinpleying a doirciii't :including "a p'h oituoe and 'amplifier 4rnay 'be 'antoniatioall' i 'reotegi for changes-'in response onaraoteristiee of the rinoto'tnoe.4 More zpartienarly, the 'intention pertains to Yn 'ietnoiis and ior raii'tornatiea1'- ly "and rperioziioally eorrejot g and 'oornpensfatinar for changes `in s'ii'si iiity of alp er systems. Generally, tne apparatus `off this :invention inyoincle's, an linterinittently veiecitiye ieeback system` connected in an inverse vor negative ieeii'oaek senseiand primarily, tnon'gf'n no ne saril'y, intended :for use in'eorreet'in-g or leonipensa'ti' 'for vserfsitiyity drift jin Ian eleotron Yrnnlt'ipiier emptier 'with or witinnit anhototnoe inoiit forming `an electro-optical system.

The present ,invention is primarily, ftnongh not necessarily, Vintention 'for nee in automatically compensating for enanges vin phototiibe `rsen'- sitvty lina scanning process, 'sp''ilb 'prooess in which photographie negatives, positives or other images Aare ed. The pre" `n't Vinteration "is desoribei and illustrated "in a "forni employing electron ninlt-i'nlier 'onototibee of -eleotroetatiealiy iooueet' L'type oornp'ri ng a "nnoto-- Vsensitive cathode 'and outont fano-tie and a pine raiity-o'releo'troetatio ffoonsing, seoonary einige sion electrodes tooniinonly known 'at ziiyirties), ont is not iiinited `thereto 'since lordinary fphotoe tubes 'may be einn'loyedi oesfirel. 'Electron mnltiplie nno'totii'bee are s'ub'ject to la Ve'ly ciisdrahl egre of srlsifvty ir'ft onring oontinneo loneration -of t'ne "tibe although f'sonie drift is diie to aging. seneitivi'ty rif't -iii'e 'to continued operation 'ie .piobalo'ly oausea "by a 'nninber oi eoon-eranA factors suon as temeerattire ychanges -oeurringuin various elemente oi the pnototnbe ieither seif an'sed by rnere opere ation `or the tiioe or externally 'caused foy 'varie ations in ambient temperatnre) and 'inne nt instability arising -'fini-rn 'the inoi'ie o'f operation of enen an eieo'tron mnltiplier phototnbeitnerein saturation entrent `flotte and 'relatively 'little space `'charge limiting 'and stabilizing effect employed. A

This eeneityity drift of electron jpj 'phototnoes nas been inajor "solved in the art ei-nee great advantages would new from 'the provision of an electron vinii'itio'lier amplifier capable of operating ina stable and tualiiy .free from sensitivity drift a niiniber of reasons, the mjo o of which iis @het 'iiti" sensitivity or amplicaton of lyt'i'ifi fmfulti- 'pliei amplii.' Also the S'b Vand Wgh't rezfola'line. (015250-267) 2 niiireinents are iniiimized. ioweifer, 'in of 'the eirtremeseneitivity and itfigh a'miiliiit of suoli onototnoee theyrnaye 'not hereto re :provento 4be eatisijaeltory for use in l'ele'etlfoy iii'c'll 'systems :intde'd foiv'use Where 'acc y important 'iinlejss V"sirenf'syetenis are operative for Aentrernely Snort erio'cis of time only, lin ii/nien oase `the time 'tor iiiriits fthe sensi vity o -t "to a relatively small valige. VA ys"e'ns'iti'vity ooinpeneateo eieotron Vrnintinlier onoto'tiioejoiiiierfof trie tine I nave invented will inn' trein-elyjtvide notification "the art, and indeed 'will forooa'oly replace moet ordinary 'ono tubes nereto ore nee-1, At the @present '-tirne I k drift eoinpen'sa'tion terne -oanabfle fperI ing the desirablef vtions oitnefsyste of t'li isresentinvention- E the operating v'con itione inet by trie rsysteni of the 'present in ntion.

Generally anfing an villustratiife preferreo iorni oi jtne `Vpresent invention eoinprieee an eleoe ilyine elettroni-o input 'to the electronic in niini-rrii'zed. In 'the oreierred iorrn of the pretent inye'ntio'nrne' are pr yideo for intermittently anti neri ital"t rentiering the inverse ffeeaoaolc ojeane respon-site to the 'iniiijtinii-er output eig-'nal 'tine vby tne effettive arnplineation of the annie ier will oe intermittentlyniodineii in a nega;

The majority of prior art negative feedback ampliiiers and closed cycle, negative feedback servomechanisms are of the continuously opererating type. That is, they employ continuous negative feedback during operation of the amplifier or servomechanism. ously operating types of negative feedback am- .pliiiers cannot attain as high a, degree of sen.-

sitvity drift compensation or correction as is possible to attain in anv intermittent negative feedback system, such as is employed in the .present invention. The reason for this is that `the amplified output of a negative feedback amplier does not precisely correspond in wave form to the input to the amplifier as would occur in an ideal system. Instead a non-linear relationship exists. I have found that this nonlinearity is practically eliminated in intermittently effective negative feedback ampliers.

In addition the method and means of the in- 'vention standardize system input modulation momentarily during each intermittent period of negative feedback, which modifies the ampliiii- Such contin-umentary negative feedback amplifier sensitivity drift correction period. This eliminates all of the above mentioned non-linearity in the relationship of amplifier output to amplifier input ina negative feedback amplifier. Thus the present invention provides virtually complete sensitivity drift correction during each intermittent rnegative feedback period to a degree of accuracy heretofore unattainable even With exceedingly complex, continuous negative feedback ampliiiers and servomechanisms.

Since the sensitivity drift in an amplier, whether of the electron multiplier phototube ;type or not, is a relatively slow thing,the inten mittent correction thereof virtually without error is much more desirable than the continuous relatively inaccurate sensitivity driftcorrection of an amplifier attainable through prior art systems and methods.

Through the use of the present invention in stability and oscillation are avoided and yet during each negative feedback period the sensitivity drift of the amplifier is virtually completely cor,- rected without error in a manner never before attainable.

i It should also be noted that the present in- .vention does not apply negative feedback in the customary manner. The negative feedback of the present invention is applied in a manner -tending to modify the amplification of the elec- ;-tron multiplier tube in a negative feedback sense, whereas most prior negative feedback systems apply the feedback to the input signal to an amplifier and do not substantially vary the vamplification of the amplifier except in rare instanceswhere variable mu tubes have been employed.

It is an object of this inventiomtherefore, to provide methods and means, including improved negative feedback systems, for virtually completely correcting for changes in response characteristics and sensitivity drift of an electron multiplier amplifier. v. I v

yA further object of the present invention is to provide an improved negative feedback system for use with an electron multiplier amplifier to intermittently apply negative feedback to the multiplier in a'manner modifying the amplification of the multiplier to intermittently correct sensitivity drift in the amplifier.

Other and allied objects will be apparent to those skilled in the art from a careful study of the illustrations, specifications and appended claims. -To facilitate understanding reference will be made to the following drawing in which:

Fig. l illustrates one embodiment of the present invention .employed in conjunction with a logarithmic amplier system of the general .type more particularly described and claimed inmy (zo-pending, allowed patent application, Serial No. 702,172, filed October 9, 1946, now Patent No. 2,454,871, and valso described and illustrated 'in my Patent No. 2,413,706 in conjunction with a mechanically driven'scanning drum adapted to carry.. a photographic negative or positive, or other image to be scanned by the photocathode ofthe electron multiplier phototube forming part of the logarithmic amplifier. j

.Fig. 2 is av graph illustrating the relationship of output current to potential difference between the ,last dynode `and the anode vof an electrostatically focused secondary emission electron multiplier tube of a type such as an R. C. A. 931 multiplier phototube.

KFig. 3 isa diagrammatic electrical schematic drawing of a second illustrative form of the present invention. 1

The example, Fig. l, shows an electron multiplier amplifier provided with means for introducing an electron-ic input thereinto and secondary emission controlling means. 'I'he specific form of electron multiplier amplifier shown in Fig. 1 is of the type wherein the electronic input is provided by a photosensitive cathode 24- of multiplier phototube l and wherein the secondary emission controlling means comprises a plu rality of secondary electron emission controlling electrodes (commonly known `as dynodes) indicated generally at 97. The output anode of the multiplier phototube l is indicated at 3. The series of secondary electron emission electrodes 97 are connected at spaced points along a resistor 4 through which a controlled electric current passes, thus applying the proper potential to each of the secondary emission electrodes 97 for controllably producing secondary electron emission from each of Said electrodes and electrostatically focusing the emitted electrons upon the succeeding secondary emission electrode. The original source of electrons is the photosensitive cathode 2 which emitselectrons in response to light input thereto, which are electrostatically focused upon the adjacent, succeeding secondary emission electrode, the secondary emission electrode immediately adjacent the anode 3 electrostatically focusing the electrons emitted therefrom upon the anode 3. One form of such electron multiplier phototube is` the R.. C. A. 931 manufactured by the Radio Corporation of America. v

The bleeder resistor 4 is connected inv series with a rectifier tube 9, terminal 6 of power supply T, and a. variable potentiometer indimatedgener-Av ally "at the opposite ends of which are connected to terminals 5 and 53 of the power sup ply Terminal 5 may be at 150 volts negative potential with respect to ground, and terminal 53 may be at 290 volts negative potential with respect to ground. The positive terminal 8 of the power supply I is grounded, thus connecting the resistor 4 in series with an electron tube 26 through the cathode 2'! thereof which is also grounded. The anode 28 of tube 26 is connected to the positive terminal 29 of a power supply 39. The `power supply 39, in the example illustrated, is of 1400 volts potential (which is considerably` greater than the potential supplied by the power supply 1) so that normally the anode of the rectier power tube 9 is negative with respect to the cathode thereof, thus allowing no current to pass through the circuit comprising the rec'tier tube 9, the resistor 4, the potentiometer' 5| and the power supply 7. The only current that normally ows through the resistor 4 is that which flows from the power supply 39 through the resistor 4 and through the potentiometer 5| through a portion of the power supply 'I to the grounded terminal 8 thereof, through the cathode 21 of the tube 29 to the anode 28 thereof and back to the positive terminal 29 of the power supply 39. Since this current flows through the tube 26 it can be seen that the current normally owing through the resistor 4 (and therefore the potentials applied to the secondary emission-controlling electrodes 91) is controlled by the bias of grid 25 of tube 28. v-

The effective overall amplification of a multiplier tube of the type illustrated at I is controlled by the potentials applied to the secondary emission controlling electrodes 91, and therefore the ampliiication of the electron multiplier phototube is controlled by the bias of grid 25 lof the electron tube 26 and also by the setting of the potentiometer 5|. However, since the potentiometer setting is fixed during normal operation of the system and is changed only during intermittent, periodic sensitivity drift correction periods it can be ignored as far as transient variations in the amplictaion of the multiplier tube I are concerned. The potentiometer 5| comprises part of the inverse feedback means and will be more fully described hereinafter.

Output anode 3 of phototube I is connected through a resistance I9 to the positive terminal 3| of a power supply 28', which is grounded at 32,. In the example terminal 3| of power supply 28', is at 399 volt positive potentiallwith respect to ground. Anode 3 is also connected to grid II of an electron tube I2, whose cathode I3 is grounded and whose anode I4 is connected through a resistance I5 to terminal 3| of power supply 2S. Anode I4 of tube I2 is also connected through a resistance I6 to the grid |8'of an elecn tron tube I9, the cathode 29 of which is grounded and the anode 2| of which is connected through a resistance 22 to the positive terminal 3| ofthe power supply 28. The lower end ofthe resistor IB and the grid I9 are connected through a resistance II to the lower terminal 3| of power supply 28' which terminal 3| is at a negative potential of 309 volts. The lower end of the resistor I9, the upper end of the resistor II and the grid I 8 of tube I9 are connected through a resistor 33 and junction 59 to inverse feedback meanswhich will be more particularly described connected through a: resistance 23 to grid 250i tube 26 and' through a resistor 24, to the negative terminal 3| of the power supply 28. The lower end of resistor 22 is also Iconnected to the anode 299 of an electron tube 29|, the cathode 292 of which is grounded and also connected to a switch 293 arranged to controllably make contact with contact 294 connected to the grid of the tube 29|. The gris and the contact 294 are also connected through a resistance 295 to a terminal 296 which is connected lto the negative terminal of a low voltage power'supply 296' (such as -15 volts).

From the above description it can be seen that the'output of the electron multiplier phototube I is resistively coupled to the succeeding amplier stage'including electron tube I2 which is resistively coupled to the succeeding amplifier stage including tube I9 which is resistively coupled to the succeeding amplier stage including the tube 25, and that the output of tube 26 (which corresponds vto the output of the multiplier tube I) controls the current flow through the 'resistor 4 and consequently the potential across the dynodes of the electron multiplier tube I, thus controlling the amplication constant thereof in an inverse feedback manner tending to maintain the anode output current of the electron multiplier tube I virtually at a constant value, irrespective of light input to the photocathode 2 of the multiplier. The Voltagel applied across the bleeder resistor 4 (which can be picked off at the output terminal 49) has a Virtually logarithmic relationship to the light input to the photocathode 2. The operation of the logarithmic amplifier is more fully decribed in the hereinabove-mentioned copending allowed patent application Serial No. 702,172, filed October 9, 1946, now Patent No. 2,454,871, and need not be described in detail herein.

I The input to the system (in the example illustrated in Fig. 1) is preferably in the form o' light modulations produced by a driven scanning drum carrying a photographic negative, positive", or any other suitable image to be scanned, which is arranged to modulate a scanning light beam produced by the lamp 4l. The mechanically drivenscanning drum and image carried thereby are not shown in Fig. 1 since they are well known in the art and are shown in the herein before-mentioned Patent No. 2,413,706.

Referencing means are provided to intermittently apply a selected reference voltage or sig nal to the secondary emission electrodes 9'! during short duration, periodic checking or correction periods. In the example this includes the tube 29| which is connected in parallelwith tube I9. Switch 293 is arranged to close under the control of motor 255 receiving power from leads 252. Motor 25| drives the scanning drum hereinabove mentioned (not shown). The closing of switch 293 removes negative bias from the grid oi the tube 29| and allows same to conduct electric current therethrough and causes grid 25 of tube 25 to become extremely negative with respect to ground and to prevent the ilow of current through the electron tube 25 and consequently through the resistor 4.

become conductive, thus applying a constant preselected reference signal from power supply 'I to the secondary emission electrodes 9? (modi-z fied,` of course, by the setting of the potentiomfeter 5|) during the intermittent periods when the switch 293 is in contact with the electric contact point 294-.

Since the` plateof tube 9 is no longer negative, tube 9 will "Motor 25| driving the scanning drum (not shown) is also arranged to close the switch 43 simultaneously with the closure of switch 203, and the image carried by the scanning drum or the scanning drum itself is provided with a strip of" constant optical density in a position such as to modify the modulation of the light originally emitted by lamp 41 to a selected reference value during the period of time when the switches 203 and 43 are closed, whereby the modulation of the` light input to the photocathode 2 of the phototube I will be of a selected reference value intermittently at precisely the same period that a selected fixed reference value potential or signal is applied to the secondary emission elec trodes 31 and at the same period that the switch 4 3 is closedto render the inverse feedback means responsive to variations in the amplified phototube output current. Y

The inverse feedback means used during the intermittent adjusting period includes an electron tube 35, the grid 34 of which is connected through a junction 53 to the output of electron tube I2. In this way the output voltage of tube I2 controls the current through tube 35 and through coil 33 of a relay indicated generally at 33. The relay coil 33 controls the position of switch arm 39 positioned between two electric contacts fili and 4I. Ii the current through tube 35 and relay coil 33 is'greater than the closing current of the relay, then connection will be made between contacts and il and the inotor will be caused to rotate in one direction. If lthe current through tube 35 and relay coil 33 :is less than the opening current of the relay then connection will be made between contacts 39 and-43 and the motor will be caused to rotate in the opposite direction. Due to the amplifying action of tubes 35 and I2 a very small voltage change at the grid of tube I2 is sufficient to change the position of the relay armature. During the intermittent adjusting period the current from anode 3 of phototube I is such as tocause a voltage drop in resistor l slightly greater than the positive potential above ground of the terminal 3l of power supply 28. in the example given the voltage of terminal 3l is +300 volts with respect to ground. If they current from anode 3 of phototube I decreases by a very small percentage from the mean value for correct adjustment, then the grid of tube I2 will be made more positive than the mean value for correct adjustment. The anode Ill of tube I2 and the grid 3Q of tube 35 will in turn be made more-negative than their respective mean values. The relay 38 will bel de-energized and cdnnection will be made between contacts 33 and 4 0. This causes the motor i2 to rotate in such a' direction that potentiometerb increases the voltage impressed on multiplier section f of phototuoe I by power supply 1. The opposite action takes place if the current from anode 3 of pliototube l is greater than the normal mean value during the automatic adjusting period.

The method. of operation of the example described hereinabove should be readily understood but may be reviewed here. It will be noted that during scanning (as for example, at the end of every other line of scan, or at any other desirable but frequently repeated time period) a numn ber of operations simultaneously take place during a momentary period of time, whereby the amplifier is checked and corrected. These siinultaneous operations include "'(al' Closure of switch 203 to `block tube 26 and interrupt the amplifier, thereby applying a fixed reference value signal to the secondary` electrodes 91 (as modified by the setting of potentiometer 5I);

(b) Modification of light from scanning lamp 41 to a selected value (as by the optical strip on the scanning drum, or other suitable means) (c) Closure of switch 43 to supply energy to` motor 42 under the control of relay 33.

Rotation of motor 42 is translated into an adjustment movement of potentiometer 5I to rnod-A ify the reference signal applied to secondary'r emission electrodes 91 in a-negative feedback sense until the multiplier tube output signal returns to the preselected standard signal. Thusl the system provides for a standard input modu-LJ- lation during the checking period, the applica-` tion of a reference signal to the secondary emission controlling means and means for modify. ing the reference signal applied to the second# ary emission controlling means in a negative feedback sense in response to deviation of the output signal from a standard value during the checking period, until such time as standard value is reached by the output signal. Thus during each checking or adjusting period the sensitivity drift of the electron multiplier amv; pliiier is virtually completely corrected.

Adjustment of the setting of potentiometer 5l by the motor 4Z during the adjusting periods acts to vary the potential applied across resistor 4 in a manner which varies the amplica-- tion of the electron multiplier tube I by varying the potential across the dynodes 91. However it also acts to vaiy the potential between the last dynode and the anode 3 of the electron multiplier phototube. As can be seen from examination of Fig. 2 when the voltage between the lastY dynode and the anode falls between certain lime` its defined by thecurves shown in Fig. 2, virtually no variation in output current of. the phototube occurs as a result of variations in the potential existing between the last dynode and the anode. Thus variations in the setting of4 :f the potentiometer 5I act to produce potential variations across the dynodes which aectthe amplification constant of the tube in such mane. ner that the voltage variations existing between the last dynode and the anode of the multiplierv tube have virtually no effect on the output cur-. rent of the multiplier tube.

- Fig;A 3 illustrates (in fragmentary form) a slight modiiication of the form of my invention shown in Fig. l wherein a variable rheostat 5I- is substituted for the variable potentiometer 5I of Fig. l. The balance of the components shown in Fig. 3 are indicated by primed reference iiurri,y erals similar to those used to indicate correspond-v ing components in Fig. 1. The operation of the device `is quite similar to that shown in Fig. l and need not be described specifically.

It should be noted that the examples described` and illustrated herein are illustrative only and numerous modifications within the scope of the present invention will occur to those skilled in the art, and all such are intended to be included and comprehended herein. For example: the secondary emission controlling electrodes need not be'simultaneously varied in potential to produce a variation in the amplification of the mul tiplierl tube. Under some circumstancs it may4 be desirable to vaiy the potential between two or moreA of the dynodes rather than across all the dynodes: Although the amplifier illustrated in Fig. 1 is of the logarithmic type, the method and means of this invention can be applied tto any amplifier employing a multiplier phototube, including an antilogarithmic amplifier of the character illustrated and described in my copending application Serial No. 55,033, led concurrently herewith.

While the present invention has been described in connection with an electron multiplier amplifier having a photocathode arranged to provide an electronic input to the secondary emission controlling means in response to light impinging the photocathode, it is to be understood that the invention is not limited to this arrangement, since any type of electronic input to an electron multiplier tube could be substituted for the photoelectric input. For example, the electronic input to the secondary emission controlling means might be of a thermionic type, or various other types of electronic input may be employed with the present invention. While the present invention is described as employing secondary emission electrodes, it is not limited to such an arrangement, since any type of secondary emission controlling means may be employed. For

example, magnetic electron focusing and/or sec .ondary emission controlling means may be ernfployed, singly or in conjunction with electrostatic secondary electron emission controlling and/ or focusing means.

The referencing means may also be modied EWithin Wide limits without departing from the spirit of the present invention. Any means for providing intermittent xed reference modulation of system input and simultaneous inverse feedback to the amplier for causing the mcdiy,

' feedback means may be employed.

The inverse feedback may be applied to the electronic input to the multiplier or to the light input to a photoelectric electronic multiplier tube rather than to the secondary emission controlling i, means, if desired. This is more particularly described, illustrated and claimed in my copending application Serial No. 55,033, led concurrently herewith.

I claim:

1. In an amplifier system including an electron multiplier phototube having a cathode, an anode, and a plurality of dynodes, a voltage supply for said dynodes and an output, the provision of means arranged to automatically correct for changes in efective amplification of the phototube, said means comprising; means for supplying light to the phototube; an inverse feedback adjustment means responsive to the output anode current of the phototube and arranged to modify the effective amplification of the phototube; means for frequently rendering the feedback adjustment means effective for periods of short duration; and means for restoring light input to the phototube to a constant value expected to produce an output anode current of selected value during said periods, said feedback adjustment means being arranged to adjust the voltage supplied to the dynodes oi the phototube by maintaining the said cathode at a constant potential and varying the potential applied to said dynodes in accordance with the deviation of the output anode current obtained during said period from the output anode current of selected value.

2. A system of the character stated in claim l, wherein the inverse feedback means includes a reversible motor; a variable resistance means operably controlled by the motor and operably connected to the voltage supply to the dynodes of the phototube; a reversing switch means for energizing the motor in either direction; and relay means, responsive to deviation of the output anode current of the phototube during an adjusting period from the output anode current of selected value, for actuating the switch means.

NORMAN R. GUNDERSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES t PATENTS Number Name Date 2,347,015 Woloschak Apr. 18, 1944 2,412,423 Rajchman et al. Dec. 10, 1946 

